Manipulation of crop plants to alter and/or improve phenotypic characteristics (such as productivity or quality) requires the expression of heterologous genes in plant tissues. Such genetic manipulation has become possible by virtue of two discoveries: the ability to transform heterologous genetic material into a plant cell and by the existence of promoters that are able to drive the expression of the heterologous genetic material.
It is advantageous to have the choice of a variety of different promoters so as to give the desired effect(s) in the transgenic plant. Suitable promoters may be selected for a particular gene, construct, cell, tissue, plant or environment. Promoters that are useful for plant transgene expression include those that are inducible, viral, synthetic, constitutive (Odell et al., 1985, Nature 313: 810-812; Granger & Cyr, 2001, Plant Cell Repo. 20: 227-234), temporally regulated, spatially regulated, tissue-specific, and spatio-temporally regulated (Kuhlemeier et al. 1987, Ann. Rev. Plant Physiol. Plant Mol. Biol. 38: 221-257). Promoters from bacteria, fungi, viruses and plants have been used to control gene expression in plant cells.
Promoters consist of several regions that are necessary for full function of the promoter. Some of these regions are modular, in other words they can be used in isolation to confer promoter activity or they may be assembled with other elements to construct new promoters. The first of these promoter regions lies immediately upstream of the coding sequence and forms the “core promoter region” containing consensus sequences, normally 20-70 base pairs immediately upstream of the coding sequence. The core promoter region contains a TATA box and often an initiator element as well as the initiation site. The precise length of the core promoter region is not fixed but is usually well recognizable. Such a region is normally present, with some variation, in most promoters. The base sequences lying between the various well-characterized elements appear to be of lesser importance. The core promoter region is often referred to as a minimal promoter region because it is functional on its own to promote a basal level of transcription.
The presence of the core promoter region defines a sequence as being a promoter: if the region is absent, the promoter is non-functional. The core region acts to attract the general transcription machinery to the promoter for transcription initiation. However, the core promoter region is insufficient to provide full promoter activity. A series of regulatory sequences, often upstream of the core, constitute the remainder of the promoter. The regulatory sequences determine expression level, the spatial and temporal pattern of expression and, for a subset of promoters, expression under inductive conditions (regulation by external factors such as light, temperature, chemicals and hormones). Regulatory sequences may be short regions of DNA sequence 6-100 base pairs that define the binding sites for trans-acting factors, such as transcription factors. Regulatory sequences may also be enhancers, longer regions of DNA sequence that can act from a distance from the core promoter region, sometimes over several kilobases from the core region. Regulatory sequence activity may be influenced by trans-acting factors including general transcription machinery, transcription factors and chromatin assembly factors.
Frequently, it is desirable to have tissue-specific expression of a gene of interest in a plant. Tissue-specific promoters promote expression exclusively in one set of tissues without expression throughout the plant; tissue-preferred promoters promote expression at a higher level in a subset of tissues with significantly less expression in the other tissues of the plant. For example, one may desire to express a value-added product only in corn seed but not in the remainder of the plant. Another example is the production of male sterility by tissue-specific ablation. In this case, a phytotoxic product is expressed only in the male tissue of the plant to ablate that specific tissue while other tissues of the flower as well as the rest of the plant remain intact. Many aspects of agricultural biotechnology use and require tissue-specific expression.
One important example of a need for promoters is for the expression of selected genes in plant roots. The plant root consists of many cell types such as epidermal, root cap, columella, cortex, pericycle, vascular and root hair forming trichoblasts, organized into tissues or regions of the root, for example, the root tip, root epidermis, meristematic zone, primary root, lateral root, root hair, and vascular tissue. Promoters isolated as root-specific or root-preferred can be biased towards promotion of expression in one or a few of these cell types. This cell-specific activity can be useful for specific applications such as regulating meristematic activity in only the meristematic cell zone or expression of a nematicidal gene in only the cell types that are contacted by the nematode pest. In other cases, broader cell-type specificity may be desired to express genes of interest throughout the root tissue. This may be useful in expressing an insecticidal gene to control an insect pest that feeds on plant roots, for instance corn rootworm (Diabrotica spp.). Broader cell-type root specificity may be accomplished with a single root-specific promoter with broad cell-type specificity or by using two or more root-specific or root-preferred promoters of different cell-type specificities for expression. A limited number of examples of root-preferred and root-specific promoters have been described. These include the RB7 promoter from Nicotiana tabacum (U.S. Pat. Nos. 5,459,252 and 5,750,386); the ARSK1 promoter from Arabidopsis thaliana (Hwang and Goodman (1995) Plant J 8:37-43), the MR7 promoter from Zea mays (U.S. Pat. No. 5,837,848), the ZRP2 promoter of Zea mays (U.S. Pat. No. 5,633,363), and the MTL promoter from Zea mays (U.S. Pat. Nos. 5,466,785 and 6,018,099). Many of these examples disclose promoters with expression patterns confined to a limited number of root tissues. Others fail to provide the root-specificity needed for expression of selected genes. Thus, there is a need in the art for isolation and characterization of new root promoters to obtain those of different breadth, expression level and specificity of cell-type expression for root-specific and root-preferred expression, particularly for root-specific expression.